356 Prof. A. Agassiz on Palceontological 



greater number of which are characteristic of the present 

 epoch. If we were to take in turn the changes undergone in 

 the arrangement of the plates of the test, as we pass from 

 Pygaster to Holectypus, to EcMnocyamus, and Echinanthidas, 

 we should have in the genera which follow each other in the 

 paleeontological record an unbroken series showing exactly 

 what these modifications have been. In the same way the 

 modifications of the abactinal and anal systems, and those of 

 the poriferous zone, can equally well be followed to EcMno- 

 cyamus, and thence to the Clypeastridaa ; while a similar 

 sequence in the modifications of these structural features can 

 be followed from Mortonia to the Scutellidee of the present 

 period. 



Passing finally to the Petalosticha we find no difficulty in 

 tracing theoretically the modifications which our early Echino- 

 conidge of the Lias should primarily undergo previous to the 

 appearance of Galeropygus. The similarity of the early 

 Cassiduloid and Echinoneoid types points to the same syste- 

 matic affinity, and perhaps even to a direct and not very 

 distant relationship with the Palaechinidae. For if we analyze 

 the Echinothurice of the present day we find in genera like 

 Phormosoma many structural features, such as the shape of 

 the test, the character of the spines, the structure of the apical 

 system, that of the poriferous zone, indicative of possible 

 modifications in the direction of Pygaster or of Galeropygus, 

 which have as yet not been taken into account. 



Adopting for the Petalosticha the same method of tracing 

 the modifications of single structural features in their palaion- 

 tological succession, w r e trace the comparatively little modified 

 pakeontological history of the Echinoneidas of the present day 

 from the Pyrina of the lower Jura. This, in its turn, has been 

 preceded by Hyboclypus and Galeropygus, while the Echino- 

 lampadae of the present day date back, with but trifling 1 modi- 

 fications, to the Echinohrissus of the Lias, itself preceded by 

 Clypeus ; and they have been subject only to slight generic 

 changes since that time, Echinohrissus being still extant, 

 while such closely allied genera as Catopygus and Cassidulus 

 of the earlier Cretaceous are still represented at the present 

 day — the modifications taking place in the actinal system, in 

 the ambulacral zones of the Echinoconidse and of the Echino- 

 lampadas showing the closest possible systematic affinity in 

 these families. Starting again from Hyboclypus, with its 

 elongate apical system, we naturally pass to Collyrites and 

 the strange Dysasteridaa, forms which, in their turn, are 

 closely allied to the Holasteridas. From Holaster on the one 

 side, and from Toxaster on the other, we find an unbroken 



